Circuit for feeding ink to an ink-jet printing head

ABSTRACT

The invention relates to a circuit for feeding ink to an ink-jet printing head. 
     Consideration is given to a circuit comprising in particular a first auxiliary circuit (R1), the function of which is to measure the ink pressure between the feed pipe (20) and the ink supply pipe (18) and a second auxiliary circuit R2 which serves if necessary to clear the orifice (41) for discharging ink drops (100). 
     This circuit can also comprise a third auxiliary circuit R3 for delivering a signal which is representative of the viscosity of the ink. 
     Measurement of the pressure and of the viscosity permits control of the principal parameters governing the operation of the printer which is equipped with the circuit. 
     The invention applies to any ink-jet printer.

The invention relates to a circuit for feeding ink to an ink-jetprinting head. It is also concerned with any printer which is equippedwith said circuit.

A number of problems arise in the ink-jet printing technique and thechief problems are recalled below. In the first place, the orificeemployed for the formation of the jet is of small size (of the order of75 microns) and there is a possibility that it may be choked by a dustparticle. It is important in this case to have a rapid means forclearing this orifice.

Furthermore, the printing qualities of a printer of this type areintimately related to the rate of discharge of ink through the orifice.The rate of discharge is liable to be modified as a result of variationin pressure of the ink upstream of the orifice and also as a result of avariation in viscosity of the ink. This latter may arise fromevaporation loss of the ink solvent which is often highly volatile.

The precise aim of the present invention is to solve all these problemsand relates to a circuit for feeding ink to a printer which is of simpleconstructional design, provides very flexible operation and makes itpossible with a minimum number of components (two pumps and fourelectrovalves) to obtain in particular a control of pressure andviscosity of the ink.

The invention is more precisely concerned with a circuit for feeding inkto an ink-jet printing head, of the type comprising an ink reservoir,the bottom wall of which has a first outlet connected to a pipe forsupplying ink to a pressurizing pump which feeds the body of theprinting head via a feed pipe 20; an inlet connected to a delivery pipewhich conveys the unused ink withdrawn at the level of a recovery troughand aspirated by means of a depressurizing pump; and a second outletconnected to a wetting pipe joined to the depressurizing pump; the feedcircuit being characterized in that it additionally comprises a firstauxiliary circuit mounted between the ink supply pipe and the feed pipeand capable of measuring the pressure existing within the feed pipe.

The invention is also concerned with an ink feed circuit which is alsoequipped with a second auxiliary circuit comprising a combination ofelectrovalves which are capable of carrying out the clearing of theinkdrop discharge orifice and producing a positive interruption of theink jet.

It is also concerned with an ink feed circuit equipped with a thirdauxiliary circuit comprising means for measuring the viscosity of theink.

It is finally concerned with a circuit of this type equipped with meansfor treatment of solvent vapors before discharging them into the ambientatmosphere.

A better understanding of the invention will be gained by means of thefollowing explanations and accompanying figures, wherein:

FIG. 1 is a schematic illustration of one example of construction of anink feed circuit for supplying ink to a printing head in accordance withthe invention;

FIG. 2 is a schematic illustration of one example of construction of adevice which is capable of measuring the viscosity of the ink andcooperates with the pressurization circuit of FIG. 1.

For the sake of enhanced clarity, the same elements are designated bythe same references in all the figures.

FIG. 1 therefore illustrates schematically a circuit for feeding ink toan ink-jet printing head 10. A circuit of this type essentiallycomprises an ink pressurization circuit and a depressurization circuitfor sucking unused ink drops 100 at the level of the recovery trough 11.

The ink is stored in a sealed reservoir 1 which, in accordance with theinvention, comprises an internal partition 22 placed parallel to theside walls of the reservoir, therefore at right angles to the bottomwall (ab) of this latter.

The ink level 38 which is always lower than the height 37 of theinternal partition 22 falls progressively from this level to the bottomof the reservoir. The bottom level 21 of the ink is detected by means ofa sensor 42. This ink is conveyed by means of a feed pipe 18 whichpasses through the bottom wall (ab) of the reservoir 1 to a pump 5 whichis driven by a motor 4. The output of the pump 5 is proportional to thespeed of rotation of the motor 4. The ink delivered by the pump 5 passesthrough a filter 8, a two-way duct 190, one way being referred-to as afeed pipe which is designated by the reference 20 and conveys the ink tothe printing head 10, the other way being referred-to as a return pipewhich is designated by the reference 19 and returns the ink through acalibrated leak 17 to the feed pipe 18.

The leak 17 makes it possible to produce a pressure drop which isproportional to the rate of ink flow through this latter.

During normal operation, the flow rate within the feed pipe 20 is whollynegligible with respect to the output flow of the pump 5 which passesthrough the leak 17. The pressure of the pipe 18 downstream of the leak17 is in the vicinity of atmospheric pressure, thus implying from theflow rate produced by the leak 17 and by the pump 5 that the pipes 19and 20 are at a pressure which is higher than atmospheric pressure andpractically proportional to the speed of rotation of the motor 4.

In accordance with the invention, a pressure sensor 9 is interposed inthe pipe 19. In one example of construction, this sensor 9 compriseselectric contacts 44 and 43 for delivering the pressure zone to bemaintained. The sensor also includes a feeler 45 which moves accordingto the measured pressure. This combination of the feeler 45 and contacts44 and 43 makes it possible to obtain a signal and to utilize anassociated electronic device (not shown) which continuously controls thespeed of the motor 4 so that the pressure within the pipes 19 and 20 ismaintained at a fixed and predetermined range irrespective of thevariation in parameters which govern it such as, for example, internalleakages within the pump or faulty calibration of the leak 17.

This first auxiliary circuit in accordance with the invention anddesignated by the reference R2 therefore ensures a stable pressure.

In accordance with another characteristic feature of the invention, adepressurization circuit cooperates with the pressurization circuitdescribed earlier, the function of which is to recover at the level ofthe trough 11 the ink drops which are emitted by the head 10 and are notused for printing.

For this purpose, there is employed a pump referred-to as adepressurizing pump 7 which is of the same type as the pump 5referred-to as a pressurizing pump. These two pumps 5 and 7 can beconnected to the same motor 4.

The pump 7 is a displacement pump which normally produces a flow ofliquid. By way of example, a gear-type pump can be employed. The inkdrops 100 which are recovered at the level of the trough 11 are suckedthrough a so-called suction pipe 27.

This result can be obtained only if the pump 7 is capable of producingan air flow, that is, as soon as the system is started-up, when thesuction pipe 27 still contains no ink.

This type of pump (gear-type pump, for example) may have internalleakages. These are negligible in the case of a liquid but arepreponderant in the case of gases.

On the other hand, a pump of this type is perfectly capable of operatingwith a gas if its internal elements remain continuously wetted by aliquid which accordingly has the effect of obstructing leakages. Inaccordance with the invention, this result is obtained by thecombination of means which is now described. The bottom wall (a, b) ofthe reservoir 1 is equipped with a so-called wetting pipe 45 whichconveys ink withdrawn from this reservoir 1 via a leak 25 to thedepressurizing pump 7, thus having the effect of continuously wettingits internal elements. Under these conditions, the pump can fullyaccomplish its sucking function, namely that of sucking air through thepipe 27. The flow rate of ink derived from the pipe 45 is limited by theleak 25 so as to ensure that it always remains below the volume flowrate produced by the pump 7. In this manner, this pump can generate apartial vacuum within the pipe 27, an extension 26 of which isclosed-off at A and intended to be connected if necessary to a devicefor measuring the viscosity of the ink as will be described hereinafter.This operation of the pump 7 takes place and this partial vacuum istherefore created even if the pipe 27 and its extension 26 contain ahigh proportion of air.

The pump 7 which is continuously wetted by ink supplied from thereservoir 1 through the leak 25 therefore sucks air and recovered inkderived from the recovery trough 11. The ink and air are discharged tothe sealed reservoir 1 via a so-called discharge duct 28 and, inaccordance with a characteristic feature of the invention, this pipe 28is connected to a rigid duct 24 which is perpendicular to the bottomwall (a, b) of the reservoir 1 and located within the compartment Cdelimited by the partition 22. This discharged ink is obviously derivedfrom the trough 11 but also from the wetting pipe 45. In consequence,even if no ink flow is recovered at the level of the trough 11, aminimum ink return flow to the sealed reservoir 1 via the rigid duct 24is maintained by virtue of the ink derived from the wetting circuit 45.The result thereby achieved is to obtain a constant level 37 of inkwhich overflows above the internal partition 22.

Air is also discharged into the sealed reservoir 1. It is evacuated tothe exterior through a tube coil 39, a nonreturn valve 47 and a pipe 46immersed in a liquid contained in a reservoir 3 which traps the solventcontained in the air before being discharged to free air through theorifice 36. In fact, the air recovered at the level of the trough 11circulates with the ink in the pipes 27 and 24 and is saturated withsolvent, especially if this latter is highly volatile. The function ofthe tube coil is to produce maximum condensation of the solventcontained in the air, whereupon the solvent returns to the reservoir 1under the action of gravity. The air which cannot be completely freedfrom all the solvent vapors therefore passes through the bubblingreservoir 3 which traps these vapors by dissolution.

If the trough 11 is obstructed, the pipe 27 attains the maximum degreeof vacuum which can be produced by the pump 7. If the printer is stoppedat this moment, the pipe 27 will suck from the reservoir 1 ink which canbe replaced only by air supplied through the orifice 36. In this case,the nonreturn valve 47 prevents any return of bubbling liquid into theink as this would entail the need for complete draining of the circuits.

The circuit R2 which provides a connection with the printing head is nowdescribed. The body 10 of this printing head which supports the orifice41 for producing the jet 100 can be pressurized as required by theoperator by means of an electrovalve 13 which is connected to the headvia a pipe 47. A so-called drain-off electrovalve 12 connects the pipe27 of the depressurization circuit to a pipe 48 which is at the samepressure as the body 10.

During normal operation, the electrovalve 13 is in the open position andthe electrovalve 12 is in the closed position.

This latter performs three essential functions. In the first place,after stoppage of the machine over an extended period of time, the pipe20 may contain degraded ink, especially if this period is of longduration. It may be useful in this case to replace this ink with freshink supplied from the reservoir 1. The ink-jet discharge rate cannotmake it possible to carry out this operation at high speed whereasopening of the electrovalve 12 makes it possible to produce a high rateof flow within the pipe 20 and within the body 10 and therefore torefresh the ink rapidly.

Secondly, the presence of this electrovalve 12 permits easy clearing ofthe orifice 41 if it is choked by a dust particle, for example. To thisend, the electrovalve 13 is closed and the electrovalve 12 is opened,with the result that the body 10 is accordingly at negative pressure.Under these conditions, it is possible to suck solvent through theorifice 41 and thus to drive the dust particle towards the pipe 48.

Thirdly, the electrovalve 12 has the function of ensuring a positiveinterruption of the ink jet during the printer shutdown procedure. Infact, when the jet is in operation, the ducts 47 and 48 which are underpressure inflate to a slight extent, especially if they are of plasticand have substantial lengths. When the electrovalve 13 is closed inorder to cut-off the jet, the residual pressure within 47 and 48 is suchthat the jet cannot be cut-off immediately by reason of its lowdischarge rate. This inevitably results in pollution of the jetenvironment, which is to be avoided. The original structural design ofthe ink feed circuit in accordance with the invention makes it possibleto overcome these disadvantages. In fact, when it is desired to cut-offthe jet, the electrovalve 12 is opened shortly before closing of theelectrovalve 13. When closing of this latter takes place, the residualpressure within the ducts 47 and 48 cannot remain as a result ofpreliminary opening of the electrovalve 12. This latter is thenre-closed shortly after closing of the electrovalve 13. The jet has beenpositively cut-off and the environment is not contaminated. The sequenceof opening and closing of the electrovalves is programmed in a knownmanner.

FIG. 2 shows diagrammatically one example of construction of a devicewhich is capable of measuring the viscosity of the ink, which isdesignated as a viscosimeter V in the remainder of the description andwhich cooperates in accordance with the invention with thepressurization circuit described earlier with reference to FIG. 1. Thisis a third circuit R3.

This viscosimeter (V) is essentially constituted by a vessel 14connected to the compartment C delimited within the reservoir 1 which isso designed as to be positioned at a higher level than the vessel 14. Itshould be recalled that the level 37 of the liquid is maintainedconstant within this compartment C. This vessel 14 is provided withthree electrodes 32, 33, 34 which extend downwards within said vessel todifferent levels and which serve to detect a bottom level and a toplevel by electrical conduction. It must be remembered that the ink isconducting. The ink will be above the bottom level (x) when theelectrodes 32 and 33 are subsequently in short-circuit; on the otherhand, the ink will be at the top level (y) when the electrodes (32) and(34) are subsequently in short-circuit.

The flow of ink through the duct 30 makes it possible to fill the vessel14; the pressure equilibrium at the liquid surfaces 37 within thecompartment C and 40 within the vessel 14 is achieved by means of a pipe35 which contains only air and solvent vapors.

Since the differences in height between the constant level 37, the endof the electrode 33 (corresponding to the bottom level), the end y ofthe electrode 34 (corresponding to the top level), the volume of thecontainer 14 and the diameter of the leak 29 are known, the filling timetr between levels x and y depends only on the rate of flow of inkthrough leak 29 and on the volume of the container 14 between theselevels. Because (i) the pressures are identical at the liquid surfaces37 within compartment C and surface 40 within vessel 14, (ii) the volumeof the container 14 is known, and (iii) and the differences in heightbetween the constant level 37, the end of the electrode 33(corresponding to the bottom level), the end of the electrode 34,(corresponding to the top level), are known, the filling time tr betweenx and y relates directly with the flow characteristics (the viscosity)of the ink.

An electrovalve 15 cooperates with the viscosimeter (V) in the mannerdescribed below. The electrovalve 15 puts the bottom of the vessel 14into communication with the point A, that is to say with the pipe 26 ofthe depressurization circuit described with reference to FIG. 1. The inkof the vessel 14 is then sucked through the duct 31, thus making itpossible to empty the vessel 14. To this end, the emptying flow ratewithin the duct 31 must be higher than the filling flow rate within theinlet pipe 30. When the ink level becomes lower than the end of theelectrode 33, the bottom level is reached, the electrovalve 15 is closedand the counter is reset to zero; the vessel 14 is again being filled inorder to proceed to another measure as follows. As the ink level, (nowmoving up) reaches the end of the electrode 33 (corresponding to thebottom level) the counter is initiated. The latter is stopped when theink level has reached the end of the electrode 34 (corresponding to thetop level) and the filling time tr in the counter is transmitted to thecontrol unit.

When the viscosity increases, in particular by reason of a loss of inksolvent by evaporation, the filling time (tr) increases. As soon as itreaches a reference value considered as a limit, the viscosity is thenstepwise corrected by successive additions of a fixed amount of solventfollowed by a viscosity measurement.

To this end, an electrovalve 16 is provided for connecting a reservesupply 2 containing pure solvent to the point A, that is to say the pipe26 which is at reduced pressure. This solvent is then delivered by thepump 7 to the reservoir 1.

It is also possible to control the operation of the electrovalves 15 and16 continuously as a function of the parameters collected at the levelof the electrodes 32, 33 and 34 of the viscosimeter V and therefore tocorrect the viscosity of the ink continuously by addition of solventwhen this is necessary.

The sealed vessel 2 is connected to the pipe 35 which permitsreplacement of the solvent by air without putting it in contact with thesurrounding air, thus offering the considerable advantage of avoidingthe risks of pollution of the environment, especially undesirable odors.It should be noted that the air which replaces the liquids employed suchas ink within the reservoir 1 and the solvent within the vessel 2 issupplied only from the recovery trough 11. The excess quantity of airwhich must exist escapes through the orifice 36 after bubbling withinthe reservoir 3. The vessels 1 and 2 are therefore at a slightoverpressure due to the height of liquid within the vessel 3.

As in the case of FIG. 1, the circuit for coupling the two electrovalves12 and 13 is provided and operates in the manner which was describedearlier.

An ink-jet printer equipped with an ink feed circuit in accordance withthe invention makes it possible to solve the principal problemspresented by this type of machine, that is to say in particular:

obtainment of a stable pressure;

control of viscosity of the ink;

clearing of the ink-jet discharge orifice;

positive interruption of the jet.

All these results are obtained by means of a simple device which is easyto utilize in practice solely by means of two pumps and fourelectrovalves. The invention applies to any type of ink-jet printer.

I claim:
 1. A circuit for feeding ink to an ink-jet printing headcomprising an ink reservoir, said reservoir having a bottom wall, afirst pipe connected at one end thereof to a first outlet in said bottomwall, a pressurizing pump connected to another end of said first pipe sothat ink is supplied thereto, communication means connecting saidpressurizing pump to said printing head for supplying ink thereto, awetting pipe connected at one end thereof to a second outlet in saidbottom wall, a depressurizing pump connected to another end of saidwetting pipe, a recovery trough positioned to receive unused ink fromsaid printing head in fluid communication with said depressurizing pumpfor drawing ink therefrom, means connecting said depressurizing pumpwith an inlet to said reservoir for conveying said unused ink to saidreservoir, and a viscosimeter placed at a lower level than saidreservoir comprising a vessel, three electrodes extending downwardlywithin said vessel, each electrode terminating at a different fluidlevel so as to detect a top fluid level and a bottom fluid level,whereby filling time between the bottom fluid level and the top fluidlevel can be determined and thereby the viscosity of the ink, and meansfor adding solvent to said viscosimeter when a reference value of theviscosity considered as a limit is attained.
 2. The circuit according toclaim 1, wherein said means for adding solvent to said vicosimetercomprises first and second electrovalves, said first electrovalve beingin fluid communication with the bottom of said vessel and saiddepressurizing pump so that when said first electrovalve is open, saidvessel is being filled, and when said vessel is draining said firstelectrovalve is then closed.
 3. The circuit according to claim 2,wherein said second electrovalve is connected between saiddepressurizing pump and a reserve tank for storing pure solvent, wherebysaid solvent is supplied to said reservoir when said second electrovalveis open.
 4. The circuit according to claim 3 wherein said reserve tankis sealed and connected to a pipe which is common to said reservoir andto said vessel so that said solvent is replaced by air from saiddepressurizing pump.
 5. The circuit according to claim 3, furthercomprising a second reservoir having an orifice therein so that excessair will escape through said orifice after bubbling through secondreservoir.
 6. The circuit according to claim 3, further comprising adischarge orifice in said printing head, and means cooperating therewithfor clearing said discharge or orifice.
 7. The circuit according toclaim 6, wherein said clearing means comprises first and secondelectrovalves in fluid communication with said printing head wherebyopening of said first electrovalve produces a substantial rate of flowwithin said printing head and thus ensures draining of ink therein, andclosing said second electrovalve and opening said first electrovalvepermits suction of solvent at the level of said discharge orifice tothereby clear said discharge orifice, and opening said firstelectrovalve prior to closing said second electrovalve prevents anyresidual pressure within the communication means between said first andsecond electrovalves and said printing head, thus insuring positiveinterruption of the jet.
 8. The circuit according to claim 7 furthercomprising means for measuring the pressure within said communicationmeans between said pressurizing pump and said printing head.